FIX: reduce amount of work onceoff does

In the past onceoff was forcing inline download of gravatars,
this can be so expensive that it will never finish

This fix ensures it only marks avatars stale which will be picked
up by regular schedules

app/jobs/onceoff/fix_invalid_gravatar_uploads.rb

@@ -2,11 +2,13 @@ module Jobs
   class FixInvalidGravatarUploads < Jobs::Onceoff
     def execute_onceoff(args)
       Upload.where(original_filename: "gravatar.png").find_each do |upload|
+        # note, this still feels pretty expensive for a once off
+        # we may need to re-evaluate this
         extension = FastImage.type(Discourse.store.path_for(upload))
         current_extension = upload.extension
 
         if extension.to_s.downcase != current_extension.to_s.downcase
-          upload.user.user_avatar.update_gravatar!
+          upload&.user&.user_avatar&.update_columns(last_gravatar_download_attempt: nil)
         end
       end
     end